In the background art methods of manufacturing attachment parts for automobiles are known, whereby from metal structural elements such as sheet-metal panel sections or the like a bodywork component of a motor vehicle is manufactured using forming steps. Afterwards, the component is often surface-treated under the effect of heat.
EP 1 041 130 A2 discloses edge flange sealing of bodywork components of motor vehicles, such as e.g. doors, tailgates, bonnets or sliding roof covers. The method used for this purpose is based on a pre-cross-linking of the sealing compound in the bodywork carcass by means of UV exposure. In a second step following immediately afterwards, the edge flange adhesive and the sealing compound are cured by the effect of heat. The bodywork components are then subjected to hot curing in a cataphoretic dip painting (CDP) oven.
Closer examination of the geometry of the attachment automobile part thus manufactured in the individual manufacturing stages reveals that the shape of the part alters considerably.
Thus, it is known that, because of the spring and/or elastic properties of the metal sheet used, during the forming steps there arise effects that are complicated to predict, particularly during flanging, pre-hemming and final hemming. Zhang, G., Hao, H., Wu, X., Hu, S. J., Harper, K., and Faitel, W., 2000, in “An experimental investigation of curved surface-straight edge hemming”, J. of Manufacturing Processes, Vol. 2 No. 4, p. 241-246 as well as Zhang, G., Wu, X. and Hu, S. J., 2001 in “A study on fundamental mechanisms of warp and recoil in hemming”, J. of Engineering Materials and Technology, Vol. 123, No. 4, p. 436-441, have conducted more thorough investigations in this respect.
Accordingly, attachment bodywork components and above all bonnets after pre-hemming and final hemming on flanging tools with a nominal geometry of the effective tool surfaces, i.e. the geometry of the tools used for this purpose corresponds to the nominal geometry of the component to be hemmed, present deviations from their nominal geometry. This is due above all to the phenomena “roll-in, roll-out, warp, recoil”.
Also, after passing through cataphoretic dip painting and after subsequent oven drying, the components again present considerable deviations from their nominal geometry. There are several possible causes of these deviations. For instance, during the non-cutting manufacture (drawing, trimming, flanging, hammering, hemming) internal stresses introduced into the bonnets are reduced. Added to this is the fact that any edge-flange and lining adhesives that are used present a different thermal expansion behavior from the metal components, which are often made of steel. Finally, curing of the adhesives gives rise to a “freezing” of the thermal-expansion-related deviation of the geometry of the bonnet at the end of the CDP cycle. The dimensional deviations arising in the course of cataphoretic dip painting and subsequent oven drying are compensated, if possible, by time-consuming and costly product- and/or process modifications as well as by optionally necessary manual straightening.